1. Field of the invention
This invention pertains to an ink jet recording apparatus having a recording head that jets an ink droplet out of a nozzle opening by displacing a pressure producing chamber by pressure using a piezoelectric vibrator so as to correspond to print data, the pressure producing chamber communicating with the nozzle opening and a reservoir. More specifically, the invention is directed to an ink droplet jetting technique.
2. Related art
An ink jet recording apparatus, such as an ink jet printer, uses an ink jet recording head to form dots on a recording medium, such as paper. In particular, the ink jet recording head forms each dot by jetting an ink droplet out of a nozzle opening of the recording head. The ink droplet is jetted out in response to a drive signal that corresponds to print data and that is supplied to the recording head. The size of the nozzle opening normally sets the size of the ink droplet and, correspondingly, the size of the dot formed on the recording medium. An ink droplet whose size is set in this manner by the size of the nozzle opening may be referred to as a normal size ink droplet.
An ink jet recording head typically includes a pressure producing chamber that communicates with both a nozzle opening and a reservoir, and a pressure producing means that applies pressure to the pressure producing chamber. This type of ink jet recording apparatus can print in full color by using different color inks to form dots of different colors.
To print graphics with photographic quality, it is necessary to make the size of a dot (i.e., the dot size) formed by an ink droplet as small as possible. One way to achieve such a dot size reduction is to reduce the area of the aperture of the nozzle opening. Reducing the size of the nozzle opening decreases the size of a normal size ink droplet, producing a better quality of printing. There is, however, a limitation as to how tiny the nozzle openings can accurately be bored.
A different way of achieving a sufficiently small dot size is proposed in Examined Japanese Patent Publication No. Hei. 4-36071. According to this proposal, a recording apparatus has an ink jet recording head with a vertical vibration mode piezoelectric vibrator as the pressure producing means. This vertical vibration mode piezoelectric vibrator is capable, first, of expanding and, then, of contracting the pressure producing chamber. Using this approach, an ink droplet is produced which has a cross-sectional area that is smaller than the size of the nozzle opening. This effect is due to the kinetic energy of the meniscus, as will now be explained.
According to this proposed approach, the pressure producing chamber first is expanded by the piezoelectric vibrator at a speed higher than during the ink charging operation, so that the meniscus close to the nozzle opening is rapidly sucked, or drawn toward the pressure producing chamber. As a result, a resonance-induced, vertically moving undulation of ink is formed on the surface of the centerline of the meniscus. When the meniscus swells, part of the ink is separated from the meniscus main body and flies, or splashes out of the nozzle opening and onto the recording medium. The thus-created ink droplet has a respective droplet size that is far smaller than that of an ink droplet with a size defined by the nozzle opening (e.g., a normal size ink droplet). Such an ink droplet may be referred to as a reduced size ink droplet. Specifically, an ink droplet whose maximum cross-sectional area ranges from about 10 to 15 .mu.m can be jetted out of a nozzle opening whose aperture ranges from 51 to 56 .mu.m. Thus, a reduced size ink droplet whose size is only about 20% the nozzle aperture can be jetted onto the recording medium.
There are disadvantages to the foregoing approach. The size of the reduced size ink droplet so created is so small, compared with the size of the nozzle opening, that many new problems arise. One problem is that a gap is disadvantageously produced between the dots formed by ink droplets that are jetted out of adjacent nozzle openings. Another problem is that, to splash an ink droplet along a predetermined route through a clearance of about 1 to 2 .mu.m between the nozzle opening and the recording medium, a certain amount of kinetic energy is required. However, the kinetic energy that the reduced size ink droplet can hold is so small that the ink droplet curves, and does not follow the predetermined path. Yet another problem is that the undulations for producing a reduced size ink droplet depend largely on the viscosity of ink, which is temperature dependent. Therefore, the reduced size ink droplet cannot stably be jetted due to the undulations being greatly effected by temperature.